Seven years in the making, the most important network standard since 100-Mbit/s fast Ethernet (100Base-T) was ratified in the summer of 1999. Now, wireless LANs are a viable complement to their wired counterparts.
Like cordless telephones, wireless Local Area Networks (LANs) offer the same functionality as their conventional-wired cousins, but with greater user mobility and a higher price. They also have a limited range, are typically slower, and don't have a comprehensive set of standards. But that won't necessarily continue to be true, once the guidelines set forth in the new IEEE 802.11 standard take hold.
IEEE 802.11 covers a number of applications, such as connecting Ethernet and Token Ring networks. The goal of this standard is to ensure interoperability among vendors of wireless LANs and to increase their use in many applications. For example, a hospital might use the same wireless LAN to serve patient-monitoring equipment, wireless notebooks for nurses, and doctor access to the patient database. It may also use the wireless LAN as a data link for technicians who maintain hospital equipment.
Public and private systems. Wireless communications function in two separate spheres: public and private. Public wireless systems depend on a network provider or operating company to provide an infrastructure. Then, it charges you to use it. These systems are comparable to the wired/switched telephone lines or private line circuits you can rent from a telephone company. Public wireless systems include cellular telephones as well as packet radio systems, designed specifically for carrying data.
Private wireless systems, on the other hand, are owned and operated separately from a telephone company or other service provider's system. You operate the wireless equipment in such a way that both ends of a connection are under your control, and there is no need to pay by the call or by the packet.
In the past, wireless links sold by a telephone company for voice or data communications operated at frequencies requiring a license. And while you had a specific frequency reserved for your use, this usually meant your license was tied to a specific location.
Rather than using licensed frequencies, the newer private wireless communications systems use frequencies reserved for Industrial, Scientific, and Medical equipment. As such, these frequencies are known as ISM bands. They are reserved at 902 MHz to 928 MHz, 2.40 GHz to 2.483 GHz, and 5.75 GHz to 5.82 GHz . IEEE chose the 2.45 GHz radio frequency band because it already received worldwide approval for wireless LAN use.
Transmitting and receiving information. The 802.11 standard defines two methods of transmitting and receiving information. Both use radio frequency (RF) transmission as the media. The first method uses a transmission technique called direct sequence spread spectrum (DSSS). The second uses frequency-hopping spread spectrum (FHSS).
DSSS technology involves changing rapidly between many different frequencies in the ISM band during transmission. By changing frequencies, a spread spectrum transmission will only interfere with another user's transmission when both happen to provide the same frequency simultaneously.
On the other hand, an FHSS system divides its operating band into channels, with transmission hopping from one channel to the next in a random sequence covering each channel in the band. To pick up the signal, receivers must match the transmitter's hop sequence both in frequency and time. Frequency hopping is only affected when the transmission hops into a "bad" or conflicting channel. When another user jams a portion of the band, it will affect the first user for a limited time only.
Selecting between DSSS or FHSS is basically a choice between the higher performance and cost of DSSS and the lower price and performance of FHSS. This performance level depends on the interference encountered and type of data traffic processed. To a certain degree, the DSSS and FHSS schemes are incompatible, since each knocks out the other's signals.
The access point and an interface. A radio frequency wireless LAN has two basic components: the access point and an interface. The access point includes the radio transmitter/receiver (transceiver) and antenna, which plugs into the wired LAN. The interface is the wireless LAN adapter, which includes the transceiver and antenna on a small PC card (PCMCIA card) for each desktop, notebook, or laptop computer.
The standard defines 1W as the maximum power level, so the distance is also limited, generally between 500 ft to 800 ft from an access point. For greater range, you can use more access points or highly directional antennas.
Interference with a RF wireless LAN depends on specific conditions. Potential causes of interference are other ISM wireless LANs, ISM devices (such as 915 MHz cordless phones), and high power adjacent channel users. Interference can also come from other equipment sources, such as microwave ovens.
Most RF systems are nominally rated at 2 Mbit/s data rate. But for all practical purposes, only the DSSS type will likely be able to handle this data transmission rate without significant performance degradation. A 2 Mbit/s data rate is satisfactory for most applications, especially if the data is sent in ASCII format. Typical response times depend on the transmission protocol.
The original article has a table that shows a comparison of delivery speeds for three different tasks, each of which are sent on a wireless LAN and Token Ring LAN.
Costs continue to come down. Not long ago, a wireless LAN cost about four or five times more than its wired counterpart. Now, the ratio is closer to 3-to-1. The average price for an access point is between $1000 and $2000, down from $1500 to $3000 just a year ago. The wireless LAN adapters, which you need for each portable PC, add most of the cost to a project. These, too, have come down in price; dipping well below $500.
Still, compared to the $150-to-$300 price range for a wired Ethernet connection, you'll have to justify most wireless LANs by improved productivity rather than equipment costs.
Other considerations. Even though the standard was under development for seven years, it wasn't until June 1996 that two manufacturers pointed out the standard did not address roaming. They proposed a roaming specification seemingly having proprietary aspects favoring their product lines. As a result, the push to provide roaming in 802.11 stalled. Currently, roaming is left to be implemented by "higher layers" in the ISO "Open Systems Interconnection" (OSI) model. Although, roaming is a requirement of 802.11, the standard does not explain implementation.
The new standard also does not provide much information on the hard-wired LAN attached to the access points. A typical 802.11 installation has a mobile PC with a wireless LAN card communicating to an access point. The access point passes the data from the mobile PC to the wired LAN, which connects to a server. However, after data arrive at the access point the 802.11 standard does not detail what happens to the data from here. This omission could lead to incompatibilities between drivers at the access point, used to convert to one of the wired LAN standards. This uncertainty has led vendors to make flexible hardware designs.
Obviously, for more demanding applications such as a multimedia program, the 802.11 standard should support higher transmission speeds. The FCC opened up two new bands in the 5 GHz area of the spectrum in January 1997. The first is in the 5.15 GHz-to- 5.35 GHz area; the second is in the 5.75 GHz-to-5.85 GHz range. Currently, we're seeing about 10 Mbit/s to 20 Mbit/s data rates in this new part of the spectrum. But look for further developments in the future, since the standard is always open for revision.
Cable performance demands. When the wireless operating frequencies were in the 150 MHz range (as was the case some years ago), installers and designers had few problems using standard coaxial cable for antenna feeders, as well as to interconnect transmitters, combiners, filters, and other components. But with the new popularity of frequencies in the 2 GHz range, the coaxial cable specification must be more stringent in terms of line losses and proper matching of connectors.
Because the attenuation of a cable increases with the increase in transmission frequency, designers have to use larger cables, accept higher losses, or specify higher-performance cables to keep signal losses to a specified minimum. As wireless LAN applications grow, however, look for manufacturers to bring new affordable, low-loss cable designs to market.
Infrared not included in standard. When the 802.11 committee was formed, there were quite a few proponents pushing to include infrared (IR) optical transmission as an accepted media. But as products came out, it became apparent infrared was not going to work well for longer distances. IR optical networks work best in same-room applications because the transmitter and receiver have to be in the same "line of sight." The IR people ceased attending the committee meetings, and IR optical networking was dropped from 802.11. It is still valid to design and install an IR wireless communications link in a Telecom network, but it will be outside of the standards requirements.
Even though it's not part of the new 802.11 standard, a designer can still use infrared as a wireless LAN option. The technology is similar to that used in the remote control device for a television set. Bandwidth isn't a problem for IR LANs; some products offer full, 16-Mbit/s Token Ring performance.
Range and reliability (transmission is adversely affected by inclement weather, including fog) are real drawbacks. A directed type IR LAN can' t penetrate opaque barriers, and it must operate on a line-of sight basis requiring that the laser diodes, or light emitting diodes, aim directly at each other.
Newer diffusion or reflection type IR LANs attempt to overcome the limitation of a directed system by focusing the light beam on a target mounted on a ceiling or wall. The signal then reflects around an open area. The diffusion method offers more configuration flexibility, but users pay a bandwidth penalty.
Other standards/specifications. If you're planning on a wireless LAN, you should know about the Wireless LAN Interoperability Forum (WLI Forum). Created in May 1996, the forum developed an open specification for the mobile market. Founded by 15 companies and currently with 22 members (principally from the United States and Japan), the WLI Forum completed its final specification in April 1997. Completely separate from the IEEE 802.11 committee (but coexisting with it), the WLI Forum provides a migration path to any future wireless LAN standard.
The WLI Forum specification focuses on a frequency-hopping spread spectrum architecture operating at a data rate of 1.6Mbit/s per channel, with 15 independent channels (hopping patterns) available. This multi-channel architecture enables up to 15 independent wireless LANs to function in the same physical space, providing up to 13 Mbit/s of aggregate network bandwidth (15 channels21.6 Mbit/s).